Bluing, you may have heard this term before, but you don't know what it is? It is the blackening of steel, which is actually a process in which the surface finish is treated. You can also call it alkaline blackening.
What is this about?
Blackening is a chemical process in which a reaction occurs, creating an oxide layer. It looks like a black surface. Basically, it doesn't have any extra important use, nor is it a necessity. It is more of a decorative nature, although it also slightly provides protection against corrosion on these treated parts.
However, in order for it to serve as protection, you must also use preservative oils.
You don't need to worry that the thickness of the material will change after blackening. Bluing does not cause such changes, it is only a very thin layer, which will not affect it.
Where is bluing used?
If you're still wondering what such a process would be good for. Then we'll tell you that it is used in the manufacture of weapons, telescopes, measuring instruments (gauges) and in the engineering industry. Such blackened surfaces can also be combined with glossy surfaces.
What are the advantages of surface blackening?
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a nicer, more stylish appearance
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protection against corrosion
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when combined with preservative oils, longer material lifespan
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abrasion resistance
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a more stable process during metal refining
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prevents oxidation
What methods of bluing do we know?
- forge blackening
otherwise also called carbonizing. You perform it by heating the object (that you need) until red hot and then immersing it in a tank of oil. There, a thin layer (of carbon) forms, which is actually imperfectly burnt oil. Once you have all this done, you must immediately, while still hot, wipe off the blackened object from the remaining oil residue on it.
- cold blackening
for this type, you will need selenious acid with copper sulfate in alcohol. Don't be alarmed, because you will get a green or gray shade. However, we do not recommend this, because materials prepared this way are highly toxic.
- other special blackening methods
here you can use various markers such as sprays or pens. They are designed to serve as active corrosion inhibitors. Which means they reduce corrosion on a specific surface.
How do you know if you did something wrong?
It can happen that this process did not go as expected, and thus the black surface turned into something else. For example, various color variants:
- gray
this appears with nitrided steels (those are the ones that are saturated with nitrogen), however, you can fix it with light corundum blasting
- red
this occurs when the steel you blued contains a lot of chromium. Again, use blasting for this, but this time with soft steel grit
- brown
this occurs when the blackening is contaminated with iron, or when you have blued for too long. However, this is the smallest damage to the surface, because you can remove it simply, using a cloth with diesel fuel.
